1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device comprising a variable resistive element whose resistive characteristic varies in accordance with application of voltage.
2. Description of the Related Art
Nonvolatile semiconductor memory devices have been applied to portable phones, personal computers, household electrical appliances, gaming devices or the like, and widely used in the industrial world. The main nonvolatile semiconductor memory device currently being utilized in the industry is flash memory. In principle, flash memory is expected to face limit of miniaturization, and thus research on new nonvolatile semiconductor memory devices that will replace flash memory has been widely carried out. Among them, a study of resistance change memory that utilizes the phenomenon that application of voltage to a metal oxide film causes resistance to change has been actively conducted recently, because the memory is more advantageous than flash memory in terms of limitation of miniaturization and because it is also capable of writing data at a high speed.
Although the study of the phenomenon that application of voltage to metal oxides such as nickel, iron, copper, titanium or the like changes resistance had been under way since 1960s (See H. Pagnia, et. al, “Bistable Switching in Electroformed Metal-Insulator-Metal Devices”, Physica Status Solidi (a), 108, pp. 11-65, 1988), then, it was never put into practical use in actual devices. At the end of 1990s, it was proposed to apply to nonvolatile semiconductor memory device the fact that by giving voltage pulse for a short time to such oxides of manganese or copper having the Perovskite-type structure, deterioration of materials can be minimized and resistance can be increased or decreased. Then, it was demonstrated that a memory array of nonvolatile unit memory devices in which variable resistive elements using these metal oxides were combined with a transistor or a diode could be really formed on a semiconductor chip. This was reported in IEDM (International Electron Device Meeting) in 2002 (See W. W. Zhuang, et. al, “Novell Colossal Magnetoresistive Thin Film NonVolatile Resistance Random Access Memory (RRAM)”, IEDM Technical Digest, pp. 193-196, December 2000), which triggered wide research to be undertaken in the semiconductor industry. Later, a similar approach was also taken in the research on oxides of nickel or copper carried out in 1960s, and memory devices produced by being combined with a transistor or diode were also reported.
All of these technologies are basically considered a same technology as they utilize resistance change in a metal oxide film to be induced by application of voltage pulse and use different resistance states as stored information in nonvolatile memory devices.
Variable resistive elements whose resistance change is induced by application of voltage, as described above, exhibit various resistive characteristics or resistance change characteristics, depending on a material of a variable resistor, that of an electrode, form and size of a device, and measurement condition. However, it is not known what causes the diversity in these characteristics. In other words, when researchers fabricated nonvolatile memory devices, they simply made operating conditions that happened to exhibit the best characteristics as a nonvolatile memory device operating conditions of that device. Therefore, the overall picture of these characteristics has not been well understood, which still leaves us without any uniform design guideline.
Such condition without any uniform design guideline indicates that the above variable resistive element has not yet grown to be an industrially applicable technology in a true sense. In other words, in the empirically optimized technology as above, although the variable resistive element described above could be used as a single nonvolatile memory device or as a component in which the nonvolatile memory devices are integrated at a small scale, it cannot be applied to modern semiconductor devices that demand high quality assurance of large-scale integration of 1 million to 100 million units as with flash memory.
Specific instances the overall picture of which has not yet been understood, as described above, include bipolar (two polarities) switching characteristic and unipolar (unipolarity) switching characteristic. The switching characteristics of the both and applications thereof have already been reported in IEDM (See W. W. Zhuang, et. al, “Novell Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM)”, IEDM Technical Digest, pp. 193-196, December 2002).
The bipolar switching implements switching between two resistance states by utilizing voltage pulses having two different polarities of plus and minus, having resistance of a variable resistive element transit from low resistance state to high resistance state with voltage pulse of any one of the polarities, and then having it transit from the high resistance state to the low resistance state with voltage pulse of the other polarity.
In contrast, the unipolar switching implements switching between two resistance states by utilizing voltage pulses having a same polarity and two different durations of long and short application (pulse width), having resistance of a variable resistive element transit from the low resistance state to the high resistance state with voltage pulse of one duration of application and then having it transit from the high resistance state to the low resistance state with voltage pulse of other duration of application.
Although so far there have been some reports on the both switching characteristics as described above, no report has done more than stating the characteristics in the operating conditions of any specific memory device fabricated.
Switching operations based on the two switching characteristics described above have benefits and problems, respectively. In fact, since the bipolar switching can implement transit time of several 10 ns or shorter as resistance increases or decreases, a memory device utilizing this can write accumulated data at a very high rate. However, due to use of application of voltage pulses of both positive and negative polarities, configuration of a circuit for implementing a semiconductor memory device becomes complex and chip size expands, thus leading to increased manufacturing cost.
On the other hand, as the unipolar switching can implement switching operation with voltage pulses of a single polarity, circuit configuration can be simplified, chip size can be smaller than that of the bipolar switching, and thus the former is better in terms of the manufacturing cost. In addition, as a combination of a diode and a variable resistive element can be used for a unit memory device, possible effect of current leakage from adjacent memory cells, which will be a problem when a memory cell array is configured as a cross point type, can be substantially reduced, thereby resulting in considerably improved electric characteristics in readout operation. However, as the unipolar switching uses two long and short voltage pulses, and, in particular, the former one needs the pulse width of a few μs, writing thereof takes 100 times longer than that of the bipolar switch. In addition, since the memory cell current during writing ranges from about several hundreds μA to a few mA as with the case of the bipolar switching, to write each memory cell, the unipolar switching also requires about 100 times as high power consumption as the bipolar switching. Thus, it is severely inferior to the bipolar switching in terms of performance during writing.
On the one hand, in terms of stability of switching operations, there exist challenges in any switching characteristics. In order to start switching operations in a stable manner, voltage pulses having optimal voltage amplitude should be selected. However, the voltage amplitude must be determined through trial and error and according to characteristics of a memory device. Thus, even in the case of the bipolar switching, stable switching operation can often be obtained by using voltage pulses to be applied that have different voltage amplitude as well as different polarities.
First, before describing the problems to be resolved by the invention and the means for solving the problems, we describe conditions that can stably implement switching operations based on the bipolar and unipolar switching characteristics as described above, as technological idea on which the present invention is founded. In other words, our description will be based on new knowledge discovered by the inventor for what enables the bipolar and unipolar switching characteristics to develop with similar materials and configuration, no theoretical explanation of stable switching operations of which has been so far given, although the phenomenon itself has been conventionally recognized.
FIG. 25 is representative of current-voltage characteristics showing basic characteristics of resistance change due to application of voltage between both electrodes in a variable resistive element that is configured to sandwich a variable resistor between an upper electrode and a lower electrode. The current-voltage characteristics as shown in FIG. 25 were measured by using a commercially available measuring apparatus (e.g., a parameter analyzer made by Agilent Technologies with the model number 4156B) that can set the upper limit (compliance) of current. Specific voltage and current values differ, depending on a material, a device structure, a manufacturing process, and a device size of individual samples to be measured. However, irrespective of a type of a variable resistor, qualitative characteristics such as those in FIG. 25 can be seen, when a variable resistor is made of an oxide metal of iron, nickel, copper, titanium or the like.
More specifically, when voltage equal to or higher than threshold voltage Va (Va+ or Va−) is applied to a variable resistive element showing resistive characteristic of high resistance state (A in the figure), it transits to resistive characteristic of low resistance state (B in the figure). The current flowing through the variable resistive element rises to the compliance value of Ic1 when applied voltage is Va or higher. Then, when the current compliance value Ic1 is set to a value that will not exceed a current value at a transit position Tb from the low resistance state (characteristic B) to the high resistance state (characteristic A), the current greater than the compliance value Ic1 does not run. Then, if applied voltage is lowered while maintaining the current value Ic1, transition from the high resistance state (characteristic A) to the low resistance state (characteristic B) will take place. Since the applied voltage after the transition to the low resistance state is lower than the threshold voltage Vb (Vb+ or Vb−) at the transit position Tb, the resistive characteristic steadily transits to the low resistance state (characteristic B) rather than going back to the high resistance state (characteristic A). Next, either when the current compliance value is set greater than the current value at the transition point Tb or when the original setting is cancelled and voltage higher than the threshold voltage Vb is applied to a variable resistive element exhibiting the resistive characteristic (B in the figure) of the low resistance state, current flowing through the variable resistive element will decrease and the resistive characteristic will transit to high resistance value (A in the figure).
If voltage equal to or higher than the threshold voltage Va continues to be applied while the variable resistive element is in the high resistance state (A in the figure), without setting the current compliance value, a transition from the low resistance state (characteristic B) to the high resistance state (characteristic A) occurs immediately after a transition from the high resistance state (characteristic A) to the low resistance state (characteristic B) because the applied voltage is higher than the threshold voltage Vb. This results in an unstable oscillation phenomenon that the resistive characteristic of a variable resistive element keeps on changing between the high resistance state (characteristic A) and the low resistance state (characteristic B). If the applied voltage is lowered in such the oscillating condition, oscillation stops when the applied voltage reaches voltage less than the higher threshold voltage Va. As the applied voltage is then higher than the threshold voltage Vb, the resistive characteristic of the variable resistive element is in the high resistance state (characteristic A), and thus no transition to the low resistance state (characteristic B) occurs even if voltage equal to or higher than the threshold voltage Va is actually applied. In sum, application of voltage to a variable resistive element alone without setting a current compliance value could not implement desired switching operations.
In addition, although FIG. 25 shows the case of resistive characteristic in which the threshold voltage Vb for the transition from the low resistance state to the high resistance state is lower than the threshold voltage Va for the transition from the high resistance state to the low resistance state, magnitude relation of these threshold voltages Va, Vb may be reversed. In such a case, although at the threshold voltage Va, a transition from the high resistance state to the low resistance state takes place in stable manner, oscillation occurs when the threshold voltage is equal to or higher than Vb, and application of voltage pulses higher than the threshold voltage Vb does not cause a transition to the high resistance state.
Thus, for a variable resistive element to perform switching operations in a stable manner, the following two conditions should be satisfied in each of the operations of transiting from the high resistance state to the low resistance state, and of transiting from the low resistance state to the high resistance state, respectively.
Firstly, when the resistive characteristic of a variable resistive element transits from the high resistance state to the low resistance state, it is necessary to apply voltage higher than the threshold voltage Va wherein the threshold voltage Va is lower than the threshold voltage Vb. Secondly, when the resistive characteristic of a variable resistive element transits from the low resistance state to the high resistance state, it is necessary to apply voltage higher than the threshold voltage Vb wherein the threshold voltage Vb is lower than the threshold voltage Va.
In a symmetrically configured variable resistive element that was reported in the past, if switching operations are performed in the variable resistive element alone, i.e., when voltage applied to the variable resistive element is turned ON and OFF under the condition that load resistance is zero or fixed to a certain load resistive characteristic, applied voltage to cause transitions between the two resistance states cannot satisfy the above two conditions simultaneously if the respective applied voltage have a same polarity. Then, in order to meet the above two conditions, it was necessary to use asymmetric nature of the bipolar switching characteristic to an asymmetrically configured variable resistive element to be discussed later or the unipolar switching operation that uses changes in the resistive characteristic due to elevated temperatures.
FIG. 26 shows resistive characteristics (current-voltage characteristics) of a variable resistive element capable of bipolar switching operation by satisfying the above two conditions. FIG. 26 shows load resistive characteristic C as well as two resistive characteristics A, B of a variable resistive element. A load circuit forms a serial circuit by electrically connecting to the variable resistive element in series, and when voltage is applied to both ends of the serial circuit, resistive voltage division of the variable resistive element and the load circuit determines voltage to be applied to the variable resistive element. In FIG. 26, voltage at each intersection of the load resistive characteristic C and the resistive characteristics A, B is voltage to be actually applied to the variable resistive element, and the intersection of the load resistive characteristic C and the voltage axis represents voltage to be applied to both ends of the serial circuit. Increasing and decreasing voltage to be applied to both ends of the serial circuit results in lateral translation (in the direction of voltage axis) of a characteristic curve or a characteristic line representative of the load resistive characteristic C. In the example shown in FIG. 26, load resistance exhibiting a linear load resistive characteristic as a load circuit is assumed in the description.
In the current-voltage characteristics shown in FIG. 26, threshold voltage VA+ that transits from the high resistance state (characteristic A) to the low resistance state (characteristic B) as a result of application of voltage to the serial circuit on the side of one polarity (positive polarity) is smaller in absolute value than threshold voltage VB+ that transits from the low resistance state to the high resistance state on the side of the same polarity (positive polarity), wherein voltage equal to or higher than threshold voltage Va+ is applied between both terminals of the variable resistive element when voltage absolute value of which is equal to or higher than the threshold voltage VA+ is applied to both ends of the serial circuit, thus causing a transition from the high resistance state to the low resistance state. The example shown in FIG. 26 has achieved similar effect to that described in FIG. 25, by substituting a load circuit for setting a current compliance. In fact, due to presence of the load circuit, increase in the current through the variable resistive element caused by a transition from the high resistance state to the low resistance state lowers voltage through the load circuit, thus automatically reducing voltage applied to the variable resistive element. If the load resistive characteristic of a load circuit is properly set, an absolute value of the voltage applied to the variable resistive element after transition to low resistance is lower than the threshold voltage Vb+ that transits the resistive characteristic from the low resistance state to the high resistance state, thus implementing stable transition from the high resistance state to the low resistance state. However, even if voltage equal to or higher than the threshold voltage VB+ of the same polarity (positive polarity) is applied to the serial circuit after transition to the low resistance state, no transition from the low resistance state to the high resistance state occurs because voltage no less than the threshold voltage Vb+ that is higher than the threshold voltage Va+ is applied between both terminals of the variable resistive element.
On the contrary, threshold voltage VB− that transits from the low resistance state (characteristic B) to the high resistance state (characteristic A) as a result of application of voltage to a serial circuit on the side of the other polarity (negative polarity) is smaller in absolute value than threshold voltage VA· that transits from the high resistance state to the low resistance state on the side of the same polarity (negative polarity), wherein voltage absolute value of which is equal to or higher than threshold voltage Vb· is applied between both terminals of the variable resistive element when voltage absolute value of which is equal to or higher than the threshold voltage VB· is applied to both ends of the serial circuit, thus causing a transition from the low resistance state to the high resistance state. If the load resistive characteristic of the load circuit common to the positive and negative polarities is set, the absolute value of voltage applied to the variable resistive element after transition to the high resistance state is lower than the threshold voltage Va· that transits the resistive characteristic from the high resistance state to the low resistance state, thus implementing a transition from the low resistance state to the high resistance state in a stable manner. However, even if voltage absolute value of which is equal to or higher than the threshold voltage VA· is applied to the serial circuit of same polarity (negative polarity) after transition to the high resistance state, no transition from the high resistance state to the low resistance state occurs because voltage no less than the threshold voltage Va· that is higher than the threshold voltage Vb· is applied between both terminals of the variable resistive element.
The point to be noted here is as follows: for a variable resistive element alone, irrespective of polarity of applied voltage, threshold voltages Vb+ and Vb· for the transition from the low resistance state to the high resistance state are respectively lower than threshold voltages Va+ and Va· for the transition from the high resistance state to the low resistance state. Nevertheless, by making a correlation (e.g., voltage difference or voltage ratio) of the threshold voltages Va+ and Vb+ asymmetrical to that of the threshold voltages Va· and Vb·, and by properly setting the load resistive characteristic of the load circuit, as threshold voltage of voltage applied to the serial circuit, it is possible to set the threshold voltage VA+ lower in absolute value than the threshold voltage VB+ on the side of positive polarity, and the threshold voltage VB· lower in absolute value than the threshold voltage VA· on the side of negative polarity. Consequently, the magnitude relation of the threshold voltages VA+ and VB+ and of the threshold voltages VB· and VA· can be reversed, thus enabling stable bipolar switching operations by applying voltage of both positive and negative polarities.
Now, the asymmetrical nature of both positive and negative polarities in the correlation of threshold voltages of the variable resistive element shown in FIG. 26 can be obtained by configuring in up-down asymmetrical manner a material of a lower electrode and an upper electrode of the variable resistive element, composition of a variable resistor, device shape, or device size or the like. In particular, implementation of stable bipolar switching may require extremely asymmetrical nature, for instance, the lower and upper electrodes being made of different materials, interface structure between the lower electrode and a variable resistor being different from that between the upper electrode and the variable resistor or the like. Excellent asymmetrical nature is easy to be occurred if rectifying characteristic such as Schottky junction is exhibited on either one of the interface between the lower electrode and the variable resistor and that between the upper electrode and the variable resistor.
However, since the conventional bipolar switching operations use voltage pulses of both the positive and negative polarities, as described above, not only the circuit configuration for implementing a semiconductor memory device becomes complex, chip size expands, and the manufacturing cost increases, but also such the structural asymmetrical nature of the variable resistive element necessitates use of different materials for the lower and upper electrodes in the manufacturing process, which thus complicates manufacturing processes and contributes to another rise in the manufacturing cost.
Aside from the bipolar switching operations to the variable resistive element of the asymmetrical structure as described above, the two conditions for conducting stable switching operations described earlier may be satisfied even by application of voltage of a same polarity if two different values are set for the duration of voltage application to the variable resistive elements.
FIGS. 27A and 27B show resistive characteristics (current-voltage characteristics) of a variable resistive element that can satisfy the above two conditions and perform unipolar switching operations. FIG. 27A shows resistive characteristics (current-voltage characteristics) of a variable resistive element when voltage pulses having short pulse width (voltage application duration) are applied, and FIG. 27B shows resistive characteristics (current-voltage characteristics) of a variable resistive element when voltage pulses having long pulse width (voltage application duration) are applied. In addition, similar to FIG. 26, FIG. 27 also shows load resistive characteristic C as well as two resistive characteristics A, B of a variable resistive element.
In the current-voltage characteristics shown in FIG. 27A, threshold voltage VAs for transition from the high resistance state (characteristic A) to the low resistance state (characteristic B) as a result of application of voltage having short pulse width to a serial circuit is lower in absolute value than threshold voltage VBs for transition from the low resistance state to the high resistance state as a result of application of voltage having the same pulse width, wherein voltage equal to or higher than threshold voltage Vas is applied between both terminals of the variable resistive element when voltage pulses absolute value of which is equal to or higher than threshold voltage VAs is applied to both ends of the serial circuit, thus causing a transition from the high resistance state to the low resistance state. Now in the example shown in FIG. 27A, similar effect to that described in FIG. 25 is obtained by substituting a load circuit for setting a current compliance shown in FIG. 25. In fact, due to presence of the load circuit, increase in the current through the variable resistive element caused by a transition from the high resistance state to the low resistance state lowers voltage through the load circuit, thus automatically reducing voltage applied to the variable resistive element. If the load resistive characteristic of a load circuit is properly set, an absolute value of the voltage applied to the variable resistive element after transition to low resistance is lower than the threshold voltage Vbs for having the resistive characteristic transit from the low resistance state to the high resistance state, thus implementing stable transition from the high resistance state to the low resistance state. However, even if voltage equal to or higher than the threshold voltage VBs is applied to the serial circuit by applying voltage pulses of same pulse width after transition to the low resistance state, no transition from the low resistance state to the high resistance state occurs because voltage no less than the threshold voltage Vbs that is higher than the threshold voltage Vas is applied between both terminals of the variable resistive element.
On the contrary, in the current-voltage characteristics shown in FIG. 27B, threshold voltage VB1 for transition from the low resistance state (characteristic B) to the high resistance state (characteristic A) as a result of application of voltage pulses having long pulse width to the serial circuit is lower in absolute value than threshold voltage VA1 for transition from the high resistance state to the low resistance state in the same long pulse width, wherein voltage absolute value of which is higher than the threshold voltage Vb1 is applied between both terminals of the variable resistive element when voltage absolute value of which is equal to or higher than the threshold voltage VB1 is applied to both ends of the serial circuit, causing a transition from the low resistance state to the high resistance state. If the load resistive characteristic of the load circuit that is common to the long and short pulse width is set, the absolute value of voltage applied to the variable resistive element after transition to the high resistance state is lower than the threshold voltage Va1 for having the resistive characteristic transit from the high resistance state to the low resistance state, thus implementing a transition from the low resistance state to the high resistance state in a stable manner. However, even if voltage equal to or higher than the threshold voltage VA1 is applied to the serial circuit, by applying voltage pulses of the same long pulse width after transition to the high resistance state, no transition from the high resistance state to the low resistance state occurs because voltage no less than the threshold voltage Va1 that is higher than the threshold voltage Vb1 is applied between both terminals of the variable resistive element.
Thus, with the same pulse width, while the resistive characteristic of the variable resistive element only transits from one to the other of the high resistance state (characteristic A) and the low resistance state (characteristic B), it cannot transit in the reverse orientation, which thus makes stable switching operations impossible. In contrast, in the conventional unipolar switching operations, through the use of application of voltage pulses having two long and short pulse width and of same polarity, a transition from the high resistance state to the low resistance state is stably implemented on one application of voltage pulse of the two different pulse width, while a transition from the low resistance state to the high resistance state can be stably implemented.
The point to be noted here is as follows: for a variable resistive element alone, irrespective of whether pulse width is long or short, threshold voltages Vbs and Vb1 for transition from the low resistance state to the high resistance state are respectively lower than threshold voltages Vas and Va1 for transition from the high resistance state to the low resistance state. Nevertheless, by making a correlation (e.g., voltage difference or voltage ratio) of the threshold voltages Vas and Vbs differ from that of the threshold voltages Va1 and Vb1 in terms of whether pulse width is long or short, and by properly setting the load resistive characteristic of the load circuit, as threshold voltage of voltage applied to the serial circuit, it is possible to set the threshold voltage VAs lower in absolute value than the threshold voltage VBs in short pulse width, and the threshold voltage VB1 lower in absolute value than the threshold voltage VA1 in the long pulse width. Consequently, the magnitude relation of the threshold voltages VAs and VBs and of the threshold voltages VB1 and VA1 can be reversed, thus enabling stable unipolar switching operations by applying voltage pulses of different pulse width.
Now it is believed that a difference in correlation between the threshold voltages Va1 and Vb1 of the variable resistive element shown in FIG. 27 due to the length of pulse width results from a change in the high resistance state (characteristic A) and the low resistance state (characteristic B) of the variable resistive element caused by changing resistance values of the variable resistive element and resistance components in the vicinity thereof due to Joule heat generated at the variable resistive element when voltage pulses of longer pulse width are applied. In particular, when voltage amplitude of voltage pulses applied to the serial circuit is fixed, generation of Joule heat will be remarkable in the case that voltage pulses of long pulse width are applied to the variable resistive element in the low resistance state (characteristic B). Thus, it is believed that characteristic change due to difference in pulse width is most obvious in the resistive characteristic in the low resistance state (characteristic B). In fact as can be seen from a comparison of FIGS. 27A and 27B, due to effect of Joule heat, the resistive characteristic in the low resistance state (characteristic B) becomes less resistive when voltage pulses of long pulse width are applied, and the threshold voltage VB1 becomes lower than the threshold voltage VBs of when the pulse width is short.
However, the conventional unipolar switching operations are disadvantageous in terms of time and power consumption needed for writing, because two types of voltage pulses of long and short pulse width should be used.
In Japanese Patent Application Laid-Open (Kokai) No. 2005-25914, there is proposed a method of implementing stable switching operations by changing voltage to be applied to gate voltage of selective transistors during programming or erasing, thereby controlling the amount of current flowing through the variable resistive elements, in nonvolatile semiconductor memory device comprising memory cells through combination of selective transistors and variable resistive elements. In this method, the amount of current flowing through the above variable resistive elements is controlled by varying ON resistance of the selective transistors connected to the variable resistive elements to be switched. However, it only provides a method of adjusting operable voltage value or resistance value when setting magnitude of voltage amplitude of applied voltage pulses to be used in changing resistance in the variable resistive elements or a resistance value of the selective transistors, and has not been successful in specifically presenting a fundamental solution to the problems of the above described conventional bipolar switching operations or unipolar switching operations. Hence, there was a need to study with large amount of labor towards optimization of materials of variable resistive elements or electrodes, a shape of a device or the like, in order to enable stable switching operations with voltage pulses having optimal voltage amplitude and pulse width in circuit designing for many purposes.